A quadrature modulator generally signifies an apparatus in which a carrier is orthogonally modulated with two modulation signals. A block diagram of a typical quadrature modulator is presented in FIG. 1. The modulator comprises two mixers 1 and 3, both of them comprising a modulation input, a local oscillator input and a high frequency output. To the mixer 1 is fed a modulation signal I as well as a local oscillator signal LO from a local oscillator signal source not shown. A modulation signal Q is fed to the mixer 3 in a corresponding way, but a local oscillator signal LO is fed to the mixer 3 through a phase shift means 2 in a 90 degree phase shift with respect to the local oscillator signal fed to the mixer 1. The high frequency outputs of the mixers 1 and 3 are connected to an adder element 4 combining the modulated high frequency signals into one high frequency signal to be transmitted forward, which signal is fed to the output OUT of the adder element 4. A typical modulation method carried out by means of a quadrature modulator of this kind is a QPSK modulation.
In a modulator, in which a carrier shall not be transmitted forward, it is difficult to balance the mixers in such a way that the level of an outgoing carrier, i.e. of a local oscillator signal leaked into a modulated high frequency signal, is small enough with respect to an outgoing signal. A local oscillator signal leak is more difficult than before, if the carrier frequency, i.e. the local oscillator frequency, is high and variable. A local oscillator signal leaked into an outgoing signal creates a power peak at the dot frequency, which power peak causes disturbances in other radio apparatuses and even disturbs the reception of the signal at the receiving end. Therefore, the authorities have occasionally given orders concerning the maximum level allowed for a carrier component leaked into a modulated signal. Additionally, a transmission of a carrier means an unnecessary power consumption, because it does not include any information.
The problem with a leaking local oscillator signal has previously been solved by trying to construct the mixers included in the modulator good enough for the frequency range used. In some cases, this can, however, be very difficult and expensive or even impossible. For instance, it depends on the inner balance of the mixers 1 and 3 shown in FIG. 1 how much the local oscillator signal LO leaks to the output of the mixer.
Further efforts have been made to correct this problem by adding direct voltages to the modulation signal I and Q, which voltages provide an outgoing signal with a constant component at the local oscillator frequency. At the manufacturing or mounting stage of the modulator, the direct voltages are set manually in such a way that the outgoing signal component at the local oscillator frequency has the same intensity as the local oscillator leak signal of the modulator, but a reversed phase. These two signals thus cancel each other out.
However, ageing of components, temperature variations and a possible change of the local oscillator frequency generally increase the local oscillator leak when the modulator is working.